Histamine is usually stored within intracellular granules in mast cells, lung, liver and gastric mucosa, etc. In response to external stimuli such as antigen binding to cell surface antibody, histamine is released into the extracellular environment. For example, when mast cells are stimulated by an antigen entering from outside, histamine is released from the mast cells and stimulates histamine H1 (H1) receptors located on blood vessels or smooth muscle to cause allergic reactions. Likewise, histamine released from ECL cells (enterochromaffin-like cells) on the gastric mucosa stimulates histamine H2 (H2) receptors on the parietal cells to promote gastric acid secretion. Based on these facts, H1 and H2 receptor antagonists have been developed as therapeutic agents for allergic diseases and gastric ulcer, respectively, both of which are now used widely as medicaments.
Further, it has been elucidated that histamine serves as a neurotransmitter and acts on histamine receptors (histamine H3 (H3) receptors) located in central and peripheral nerves to thereby exert various physiological functions. This receptor was cloned in 1999 and determined for its gene sequence and amino acid sequence. However, its amino acid sequence homology was as low as 22% and 21.4% with H1 receptor and H2 receptor, respectively (see Non-patent Literature 1). H3 receptors are present in the presynaptic membrane and are shown to serve as autoreceptors controlling the synthesis and release of histamine (see Non-patent Literature 2). Moreover, H3 receptors are also shown to control not only the release of histamine, but also the release of other neurotransmitters including acetylcholine, serotonin, dopamine and noradrenaline (see Non-patent Literature 3). These facts suggest that selective H3 receptor modulators may serve as therapeutic agents for various diseases related to abnormal release of neurotransmitters in the nerves.
In fact, the results of animal model studies using synthetic compounds indicate a possibility that H3 receptor antagonists or inverse agonists can be used as therapeutic agents for dementia, Alzheimer's disease (see Non-patent Literatures 4 and 5), attention-deficit hyperactivity disorder (see Non-patent Literature 6), schizophrenia (see Non-patent Literature 7), epilepsy, central convulsion, etc.
Moreover, it is shown that H3 receptors are involved in eating behavior (see Non-patent Literature 8); and hence possible target diseases for H3 receptor antagonists or inverse agonists also include metabolic diseases such as eating disorders, obesity, diabetes, hyperlipidemia, etc.
Further, it is shown that histamine regulates the circadian rhythm in the brain and is responsible for maintaining a balance between waking and sleeping states (see Non-patent Literatures 9 and 10); and hence possible target diseases for H3 receptor antagonists or inverse agonists also include sleep disorders and diseases associated with sleep disorders such as narcolepsy, sleep apnea syndrome, circadian rhythm disorder, depression, etc.
Furthermore, it is shown that H3 receptors are present in sympathetic nerves on the nasal mucosa, and there is a report showing that the combined use of H3 and H1 receptor antagonists remarkably improves nasal congestion (see Non-patent Literature 11). This indicates a possibility that H3 receptor antagonists or inverse agonists are useful for treatment of allergic rhinitis or other diseases, either alone or in combination with H1 receptor antagonists.
H3 receptor antagonists or inverse agonists have been summarized in several reviews (see Non-patent Literatures 12 to 15), and reference may be made to these reviews. In the early years, many reports were issued for imidazole compounds starting from histamine itself as a leading compound. However, these compounds have not yet been developed as medicaments because they are feared to have negative effects such as inhibition of a drug-metabolizing enzyme, cytochrome P450 (CYP).
In recent years, many documents and patents have been reported for non-imidazole H3 receptor antagonists or inverse agonists (see Patent Literatures 1 to 10).
Moreover, histamine H3 receptor antagonists having a dihydroquinolinone structure have also been reported (see Patent Literature 11). However, there is no report about compounds having the structure disclosed in the present invention. As to compounds having a dihydroquinolinone skeleton, hypoxia improvers, platelet adhesion inhibitors and antiarrhythmic agents have been reported (see Patent Literatures 12 to 14). However, there is no disclosure about their affinity for H3 receptors or their selectivity toward histamine receptor subtypes.